cc : Fix occlusion tracking bug for contributing surface

cc/trees/occlusion_tracker.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/occlusion_tracker.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 30 matching lines @@
   if (stack_.size() < 2)
     return Occlusion();
   // A contributing surface doesn't get occluded by things inside its own
   // surface, so only things outside the surface can occlude it. That occlusion
   // is found just below the top of the stack (if it exists).
   const StackObject& second_last = stack_[stack_.size() - 2];
   return Occlusion(draw_transform, second_last.occlusion_from_outside_target,
                    second_last.occlusion_from_inside_target);
 }
 
+const RenderSurfaceImpl*
+OcclusionTracker::OcclusionSurfaceForContributingSurface() const {
+  // A contributing surface doesn't get occluded by things inside its own
+  // surface, so only things outside the surface can occlude it. That occlusion
+  // is found just below the top of the stack (if it exists).
+  return (stack_.size() < 2) ? nullptr : stack_[stack_.size() - 2].target;
+}
+
 void OcclusionTracker::EnterLayer(const LayerIteratorPosition& layer_iterator) {
   LayerImpl* render_target = layer_iterator.target_render_surface_layer;
 
   if (layer_iterator.represents_itself)
     EnterRenderTarget(render_target);
   else if (layer_iterator.represents_target_render_surface)
     FinishedRenderTarget(render_target);
 }
 
 void OcclusionTracker::LeaveLayer(const LayerIteratorPosition& layer_iterator) {
@@ skipping 323 matching lines @@
   DCHECK(layer_tree->RootRenderSurface() == stack_.back().target);
   const SimpleEnclosedRegion& occluded =
       stack_.back().occlusion_from_inside_target;
   Region visible_region(screen_space_clip_rect_);
   for (size_t i = 0; i < occluded.GetRegionComplexity(); ++i)
     visible_region.Subtract(occluded.GetRect(i));
   return visible_region;
 }
 
 }  // namespace cc